1. Field of the Invention
The present invention relates to color processing for adjusting a profile used to convert color data dependent on a source device into those dependent on a destination device.
2. Description of the Related Art
A color space to be handled by a device such as a monitor or printer, which outputs a color image, is related to that device. For this reason, when a device is controlled to output a color image, image data of that color image has to be converted into that on the color space of the device.
Devices having different types have color gamuts with different shapes and sizes. Therefore, when a color image reproducible by a certain device is to be output by a device which has a narrower color gamut than the former device, a color image with the same colors is not always reproduced. For this reason, color conversion such as gamut mapping is required to allow a device with a narrow color gamut to reproduce colors that cannot be reproduced by that device.
As such color conversion, various methods are available, and color conversion processing is designed for each condition that combines the use application of a color image, the types of device and output medium, viewing conditions of the color image, and the like. The color conversion processing is described in a profile associated with the condition. The profile describes a conversion table, conversion matrix, or conversion function (to be referred to as a color conversion table hereinafter), which represents the relationship between color signals of a source device (to be referred to as input signals hereinafter) and those of a destination device (to be referred to as output signals hereinafter). Upon outputting a color image, by designating an appropriate profile, color conversion processing described in that profile is applied to image data.
In color conversion processing using a profile, if the conversion precision of a color conversion table is insufficient, an ideal color conversion result cannot be obtained. Furthermore, although a color conversion table immediately after creation has sufficient conversion precision, the device characteristics may change due to aging of a device, and a satisfactory color conversion result may not often be obtained. In such case, it is required to re-create or adjust the color conversion table.
When an output image is evaluated, a specific color on an image such as the color of a logotype, or the skin color of a portrait is taken into consideration. Most of logotype colors are spot colors, which are different depending on companies that possess the logotypes. Memory colors represented by a skin color have personal differences. That is, it is difficult to uniquely decide color conversion that obtains color reproducibility preferred for all users. Hence, the color conversion table is adjusted so that more preferable color reproducibility is obtained for each user, and the color reproducibility of colors on which the user attaches importance (important colors) is satisfactory for that user.
As means for adjusting the color conversion table so as to obtain color reproducibility the user desires, a color tone modifying apparatus disclosed by Japanese Patent No. 2740436 is available. With this apparatus, the user inputs, as an adjustment instruction, a color to be modified, an effective range of color modification, and a target value as an adjustment goal of the color to be modified. Then, the apparatus adjusts the color conversion table so that an output color signal corresponding to the color to be modified matches the target value. In this case, designation of the effective range of color modification is facilitated by expressing it by lightness L, saturation S, and hue H as three attributes of a human perceived color. Normally, the color conversion table, which converts color signals of a source device into those of a destination device, does not include any description indicating the correspondence between device colors and perceived colors. For this reason, the invention of Japanese Patent No. 2740436 includes a lookup table (17 in FIG. 2) used to calculate CIEXYZ values from RGB values, CIELab values from the CIEXYZ values, and HSL values from the CIELab values.
The invention of Japanese Patent No. 2740436 inputs the target value as an output signal value. Therefore, the user has to recognize a color signal value on the color space of an output device, which expresses the target value of the color to be modified. The device characteristics are different for respective devices, and the user is required to have a good deal of knowledge about that device so as to appropriately designate the target value as the color signal value of the output device.